1. Field of the Invention
This invention lies in the field of rubber articles, particularly those formed by dip-molding. In particular, this invention addresses methods of vulcanization of dip-molded rubber articles.
2. Background of the Invention
Natural rubber latex has been extensively used as a material of construction for elastomeric dip-molded medical devices and medical device components. Examples of medical devices and components made from natural rubber latex are surgical gloves, examination gloves, finger cots, catheter balloons, uterine thermal ablation balloons, catheter cuffs, condoms, contraceptive diaphragms, indwelling urinary drainage catheters, and male external urinary drainage catheters. Other examples will be apparent to those skilled in medicine and in the manufacture and use of these and similar medical devices. Dip-molding techniques are also used in making elastomeric devices non-medical uses. These include toy balloons, industrial gloves, household gloves, and other similar devices. These devices, both medical and non-medical, can also be formed from synthetic rubber latex materials rather than natural rubber. In some cases, synthetic materials are preferred, for example where natural rubber is deemed unsuitable for some reason or where the synthetic material offers an advantage.
In latex dip-molding processes, dip formers are dipped in a latex bath, then withdrawn from the bath, dried in hot air, and vulcanized in hot air. In some cases, the latex is pre-vulcanized, i.e., the rubber particles in the latex are partially or fully vulcanized prior to the dipping step. A prevulcanized latex produces a film with improved wet and dry gel strengths, and when further vulcanization is performed after dipping and hot air drying, the tensile properties are improved. An advantage of prevulcanization is a reduction in the process time by lessening or eliminating the time required for the post-dip vulcanization. In some dip-molding processes, a chemical coagulant is included in the latex or on the dip former, and heat-sensitized coagulant dipping methods are applied to produce articles have a greater film thickness. Multiple dips are also used in some processes. Details of these and other methods are well known to those skilled in the art of latex dip molding. Further descriptions of the process and its variations are found in Pendle, Dipping with Natural Latex, published by The Malaysian Rubber Producers"" Association (1995).
Vulcanization performed on the latex film after the dip former is removed from the bath serves to form covalent bonds both within the individual rubber particles and between coalesced rubber particles. A problem with vulcanization both at this stage and prior to the dip is that the outer surfaces of the particles have greater exposure to the vulcanizing agents than the particle interiors, resulting in a case-hardening effect and a lack of uniformity in the rubber.
In dip-molding processes for rubber latices, sulfur is the primary vulcanizing agent, although various accelerators, activators, sulfur donors, and boosters are frequently included as well. A description of prevulcanization methods and formulations for both natural and synthetic rubber latices is found in Blackley, D. C., Polymer Latices: Science and Technology, 2d Edition, Vol. 2, Chapter 13 (Chapman and Hall, 1997). Prevulcanization methods performed without sulfur are those utilizing free radical crosslinking, which can be achieved by various means, including high energy irradiation in the presence of a chemical sensitizer. Natural latex prevulcanized in this manner is referred to as xe2x80x9cradiation vulcanized natural rubber latexxe2x80x9d (RVNRL). Descriptions of such latices and the vulcanization processes used in their preparation are found in Zin, W. M. B. W., xe2x80x9cSemi industrial scale RVNRL preparation, products manufacturing and properties,xe2x80x9d Radiat. Phys. Chem., 52(1-6), pp. 611-616 (1998).
Rubber films from RVNRL are produced by simply casting the latex into films and then drying the films. No vulcanization is done after the film is cast, and none can be done unless curative agents are subsequently added. Films made by this process have tensile strengths of up to 27.1 megapascals (3930 psi). While this meets the requirements of many dip-molded rubber devices, such as surgical gloves for example, the tensile strength of these films is not as high as that achieved in many sulfur-vulcanized films where a post-vulcanization step (after the dip stage) is included. The RVNRL films are also lower in the value of the 100% tensile modulus than sulfur-vulcanized films. The RVNRL films also suffer from a lack of any means to achieve true particle integration by covalent bonds. This makes it difficult to form a truly integrated, pore-free latex rubber film from RVNRL. A further disadvantage is the need for access to an irradiation facility, which may not be in a location that is convenient to many rubber manufacturers and which adds considerably to the cost of manufacture.
An alternative means of prevulcanization of latex by free radical crosslinking is that which involves the use of organic peroxides and hydroperoxides. Latex that is prevulcanized with these materials is referred to as xe2x80x9cperoxide vulcanized natural rubber latexxe2x80x9d (PVNRL). Descriptions of such latices and methods for preparing them are found in U.S. Pat. No. 2,868,859, issued Jan. 13, 1959, to G. Stott, entitled xe2x80x9cCuring Natural Rubber Latex With a Peroxide.xe2x80x9d The process disclosed in this patent involves superheating natural rubber latex in the presence of 2% (based on dry rubber weight) ditertiary butyl peroxide in a pressure vessel at a temperature of 170xc2x0 C. for fifteen minutes. The latex was then cooled, and the films cast and dried to yield vulcanized rubber films with a tensile strength as high as 251 kg/cm2 (3739 psi). The film was formed simply by drying, with no post-drying vulcanization. Unfortunately, utilization of this process on a commercial scale would require large and expensive heated pressure vessels, and prevulcanization is a necessary part of the process.
Latex prevulcanized with a hydroperoxide rather than an organic peroxide is described in U.S. Pat. No. 2,975,151, issued Mar. 14, 1961, to W. S. Ropp, entitled xe2x80x9cVulcanization of Latex With Organic Hydroperoxide.xe2x80x9d In this patent, natural rubber latex is prevulcanized by superheating under pressure at 250xc2x0 F. (121xc2x0 C.) for about one hour with cumene hydroperoxide. The resulting cooled latex is cast into a film, then air dried. The product had a maximum tensile strength of 2775 psi. As in the Stott patent, the utilization of this process on a commercial scale would require large scale heated pressure vessels, and the tensile strength is not nearly as good as that of a sulfur-vulcanized latex or of the organic peroxide prevulcanized latex of Stott.
The use of hydrogen peroxide as a prevulcanizing agent with an activating chemical is disclosed in U.S. Pat. No. 3,755,232, issued Aug. 28, 1973, to B. K. Rodaway, entitled xe2x80x9cVulcanization of Latex With Organic Hydroperoxide.xe2x80x9d The method of this patent is performed at lower temperatures without the use of pressure vessels. The patent cites an example in which a natural rubber latex is prevulcanized by this method, cast into a film and dried, to yield a product with a tensile strength of 124 kg/cm2 (1760 psi). Thus, despite its advantages this process produces latex films of interior strength. The possibility of adding a sulfur curative system to the latex after prevulcanization to permit post-casting vulcanization is suggested, but this would involve the use of sulfur curative chemicals, which peroxide processes are generally intended to avoid. In further examples, curing of polychloroprene and other synthetic latices is performed with hydrogen peroxide and an activator, the products in each case having inferior tensile properties.
Further disclosure of technology forming the background of the present invention is found in U.S. Pat. No. 3,892,697, issued Jul. 1, 1975, to O. W. Burke, entitled xe2x80x9cPreparation of Crosslinked Polymer Latex From Aqueous Emulsion of Solvent/Polymer Solution of Precursor Latex Particle Size.xe2x80x9d In the process disclosed in this patent, dicumyl peroxide is mixed with a synthetic polyisoprene latex under 6000 psi pressure, and the mixture is subjected to an unspecified elevated temperature for an unstated period of time. There is no disclosure of film formation.
Still further methods forming the background of the invention are those known as xe2x80x9ccontinuous vulcanization in liquid bathsxe2x80x9d (LCM Vulcanization) which are used on extruded rubber profiles. In LCM Vulcanization, a solid constant profile shape is extruded, then submerged in a hot liquid bath such as molten salt, hot oil, or melted lead, or in a hot fluid medium such as fluidized sand particles. Essentially all molecular oxygen is excluded from the curing environment. The use of the hot liquid bath or fluid medium is to provide very rapid heat transfer rates to thin-wall extruded rubber profiles. Descriptions of various LCM Vulcanization methods are found in Hoffman, Rubber Technology Handbook, pages 394-398 (Hanser Publishers, 1994), and in U.S. Pat. No. 4,981,637, issued Jan. 1, 1991, to M. L. Hyer, entitled xe2x80x9cMethod of Forming an Improved Wiper Blade.xe2x80x9d These references do not disclose application of the process to dipped films.
Latex articles formed by dip molding must be pore-free if the passage of pathogens or other unwanted substances through the article walls is to be prevented. Pore-free walls require good integration and adhesion between the rubber particles of the latex. Many attempts have been made to achieve this, but it remains a difficult goal. Excessive vulcanization for example tends to inhibit particle integration. A simple means of determining the extent of prevulcanization is a test known as the chloroform coagulation test. A description of this test can be found in The Vanderbilt Latex Handbook, 3d Edition, page 110 (R. T. Vanderbilt Company, Inc., Norwalk, Conn., USA).
All patents and publications cited in this specification are incorporated herein by reference.
It has now been discovered that pore-free rubber articles can be prepared by dip-molding processes by including vulcanizing agent(s) in the dipping medium and vulcanizing the wet film by immersing the dip former in a heated liquid bath that is chemically inert. The temperature of the heated bath will be sufficiently high to cause at least a partial melting and/or softening of any coalesced rubber particles in the film while vulcanizing the film, and the time needed to effect vulcanization under these conditions is considerably less than that typically used for vulcanization in hot air. The resulting film is coherent and essentially pore-free.
Another aspect of this invention resides in the discovery that the tensile properties of an article of vulcanized rubber can be improved to an unusually effective degree by immersing the already vulcanized article in a solution of a vulcanizing agent to cause the rubber of the article to absorb or imbibe the vulcanizing agent from the solution, and then immersing the rubber and the imbibed vulcanizing agent in a heated liquid bath that is substantially free of molecular oxygen and chemically inert. After recovery of the article from the bath, the tensile properties are considerably greater than those that the product would have if the same amount and type of vulcanizing agent were included in the original vulcanization.
This invention is useful in the manufacture of articles of all rubber materials, both natural and synthetic. For certain aspects of this invention, notably those that reside in the use of the heated liquid bath for a single-stage vulcanization after dip-molding, the preferred rubber materials are those other than cis-1,4-polyisoprene.
Among the many advantages of this invention is a faster vulcanization rate without the risk of undesirable oxidation of the dipped parts. The invention also offers superior particle integration and thus more coherent latex films by partially melting the particles as they are being crosslinked and heating them more thoroughly, which reduces the tendency of the particles toward case hardening. Prevulcanization, i.e., vulcanization performed on the dipping liquid prior to the dip stage, can be eliminated in many cases, and this offers advantages for latices that are peroxide cure systems or sulfur cure systems where prevulcanization is used in part to reduce the cure times and to reduce the quantity of nitrosamines that may be released during dip molding operations. Alternatively, the postvulcanization of the invention, referring to its occurrence subsequent to the dip stage, can improve the tensile properties of the product without the need for the addition of sulfur-based chemicals. Postvulcanization can also be performed using a different reaction than that used for the prevulcanization. For example, postvulcanization with the use of peroxides can be performed on latices that are prevulcanized by sulfur, by peroxide, or by radiation. In systems that are susceptible to nitrosamine formation, the invention reduces or eliminates the amount of nitrosamines that are formed. When polychloroprene latices, nitrile latices or mixtures of the two are used, postvulcanization by use of organic peroxides can be achieved with very small amounts of the peroxides. With peroxide-based and radiation-based vulcanization systems, the use of the present invention provides products with a 100% tensile modulus that is higher than has been previously obtained with such systems, and yet with no loss of tensile strength. With sulfur-based systems, the high-temperature, oxygen-free environment helps to prevent the degradation that is caused by hydroperoxides that are typically generated during hot air vulcanization. Such degradation is responsible in part for the aging of latex. Use of the invention in non-sulfur-containing systems such as peroxide-based systems results in products with a longer shelf life. Still further, the elimination of the need for hot air curing and its inherent inefficiencies offers considerable savings in energy, since hot media baths are easily insulated. Further energy savings are also available when prevulcanization and maturation are eliminated. The means by which these and other objects and advantages are achieved, as well as particulars of the process and its preferred embodiments, will be evident from the description that follows.